Spatial analysis of mixed-conifer seedling dispersal in contrasting regeneration environments

Background/Question/Methods Mixed-conifer forests of the Sierra Nevada underwent a marked shift to dominance by shade-tolerant trees during the 20^th century, but the concomitant emergence of a disturbance regime of large high-intensity fires may now provide opportunities for establishment of shade-intolerant pine species. We assessed conifer seedling establishment in stem-mapped tree stands representing three regeneration environments: isolated remnant patches nine yr after high-intensity fire, contiguous stands nine yr after low-intensity wildfire, and silvicultural openings ~1 ha in size. We asked how seedling density varied as function of distance from parent tree, and how seedling establishment varied among substrate type (e.g., conifer litter compared to bare soil), and how substrate availability varied among regeneration environments. Seedling density was estimated as a function of distance from parent tree and substrate favorability, assuming a log-normal dispersal curve.

Results/Conclusions

Only ponderosa pine and white fir were sampled in sufficiently high numbers to allow estimation of dispersal parameters. Modal ponderosa pine dispersal distance was 45 m, almost twice the distance of white fir. Ponderosa pine seedling distribution was narrowly clustered around the modal dispersal distance, possibly because of dispersal limitation from large seed size. White fir, which has smaller seeds, maintained high dispersion densities at relatively long distances from individual trees. Ponderosa pine seedlings established preferentially in areas of high herbaceous cover, and white fir preferred bare soil. Conifer litter was the most common substrate by a wide margin in all regeneration environments, and even though it was not the most favorable substrate for either species it accounted for a high proportion of seedling occurrences. Group selection openings had a larger proportion of bare soil than the other environments (25% of soil surface vs. 12-15% in burned areas), which was associated with dense white fir regeneration. We conclude that adequate regeneration of the shade-intolerant ponderosa pine was occurring in group-selection openings, but insufficient natural regeneration of pine was occurring in the post-fire environments to effect substantial change in relative abundance of shade-intolerant species.